Optical fiber communications is one of the drivers to enable broadband services to be delivered by an operator to the customers that can be spread over larger geographic areas. Optical fiber is used as transmission medium because it offers several advantages compared to the copper wires, such as the traditional twisted pair. Fiber-to-the-X (FTTx) technology (X can stand for Curb, Node, Building, Home or other) has been extensively studied worldwide, for delivering high bandwidth to users and for converging wireless and wireline.
An important point for FTTx is the capability for building future-proof broadband networks with low installation and operating expenditures. While active optical networks (AONs) exist, taking advantage of repeaters and switches for reach extension and routing, passive optical networks (PONs) are also gaining attention due to the fact that no active components are deployed in the distribution plant between the operator and the customers. In this way, cost deriving from maintenance of active devices can be kept low as they are situated either at the central office of the provider or located at the customer premises.
The capacity and number of served users can be expanded by taking multiplexing technologies into account in the architecture of the access network (AN), regardless if it is of active or passive nature. As the optical fiber is suitable to transmit on multiple optical frequencies, wavelength division multiplexing (WDM) can lead to a significant improvement in cost and capacity, as fiber infrastructure can be shared between the customers while more data signals can be transmitted on different wavelengths. This kind of AN has an optical multiplexer situated between the operator and its customers, and is herein referred to as a WDM-AN.
Furthermore, each wavelength can, for the case of a so-called hybrid AN, be divided into time slots by means of time division multiplexing (TDM) for splitting the signal in its power to a bunch of customers instead of only one. Although this procedure leads to a reduction in the data rate per user, the naturally high data rates that can be achieved for each wavelength, thanks to the maturity of optical transmitters, ensures that the net data rates for the single customers still stay high. This kind of AN has, in addition to the multiplexer of the WDM-AN, a power splitter located at each output of the multiplexer, while the customers are connected to the outputs of the power splitter. Such an AN is herein referred to as a WDM/TDM-AN.
Expanding the AN by multiplexing means that the cost can be reduced due to a shared infrastructure at the fiber distribution plant and also at the central office of the network operator, referred to as the optical line terminal (OLT) herein, where several light sources and expensive equipment such as modulators and devices for signal conditioning are located. One requirement for introducing multiplexing into the AN is to keep the customer premises equipment, referred to as the optical network unit (ONU) herein, identical and therefore agnostic to these multiplexing techniques. A reflective design without active optical source or a design with a tunable optical source allows to have one single ONU, which can be used at any position inside the AN (i.e. the ONU is operable with different wavelengths and at different ports or a power splitter). Such a design that is suitable for mass deployment of ONU ensures cost effectiveness as the ONU will determine the expenditures for an AN with a high number of users.
The use of reflective modulators integrated together with optical amplifiers at the customer premises is a promising solution for the ONU. In this way, the loss over the network can be overcome while imprinting upstream transmission data on the incoming signal. An efficient AN uses a single wavelength as optical input signal for an ONU, which carries the data transmission from the OLT, referred to as the downstream, and also the data transmission from the ONU back towards the OLT, referred to as the upstream. These two data streams are present at the same time due to the bidirectional nature of communication.
Realistic deployment of the access networks discussed above, require ONUs that are not wavelength-dependent (color-agnostic or colorless) and are capable of re-using the same downstream signal wavelength for modulating the upstream data. Re-modulation of downstream can be efficiently done by using orthogonal modulation formats, as they avoid crosstalk between down- and upstream. However, the design of the ONU becomes more complicated as more complex modulation formats have to be used (compared to the simplest intensity modulation format), which may prevent a cost-effective deployment of customer premises equipment. This disadvantage is mainly motivated by the inability of photo-detectors to acquire information from the optical signal such as phase or frequency in addition to its intensity. Therefore, modulation formats that imprint data in the phase or frequency of the optical signal require in principle additional components such as filters or other information-converting structures [Prat05] [Martinez08].
Promising candidates for a reflective modulator, which can take the advantage of just having to modulate the intensity of the constant-envelope downstream signal, are the reflective semiconductor optical amplifier (RSOA), the reflective electro-absorption modulator or integrated versions of semiconductor optical amplifier (SOA) and reflective electro-absorption modulator (REAM), where the SOA acts as amplifier to overcome also the losses of the REAM or any reflective active optical component capable of intensity modulating the upstream data signal.